the manuscript has been favourably assessed by two reviewers. Please submit a suitably revised manuscript along with a point-by-point reply as to how the manuscript has changed in response to the reviewer comments. Please do make sure that your responses to both reviewer #1 and #2 comments are adequately reflected in the revised manuscript.

Please also ensure consistent use of OCS throughout the manuscript, and in particular in tables and figures - this is not always the case, and I find the concurrent use of COS and OCS confusing. Please also attempt to improve the legibility and quality of the figures, and avoid the impression that they are copy-pasted from a selected range of presentations.

many thanks for your revised version, and apologies for the delay in coming to a decision, which is owed to the length of the manuscript. I believe that you have addressed most of the reviewer comments adequately - with some minor exceptions, and notably the omission of providing updated figures opposite to what was said in the response letter.

That said, I believe that it is possible and necessary to cut the length of the manuscript by about a third without loosing information if this is a review to be read and understood, and not only cited. This is because of i) some sections provide irrelevant detail lacking the synthesis character the “Review and synthesis” category is designed for; and ii) the text contains numerous redundancies across and within subsections and unnecessary sentences, which hampers reading a lot. A thorough editing of the manuscript to ensure consistency in treatment, content and layout of the sections across the different sections and to reduce redundancy seems paramount to me before this manuscript can be published in Biogeosciences.

I further do not understand the current structure of the manuscript, which presents available measurements AFTER the application of OCS. It would seem to me more logical to have Section 4 prior to Section 3, but I’m open to your ideas on this.

I would also appreciate if – especially in section 2 – would have a common format suggestion recommendations at the end of each section, possibly starting with “Recommendations: “

Below, I give examples of redundancies and unnecessary sentences below, but this list is by far not complete. Please spend sufficient time in making this a concise and homogeneous manuscript.

Best wishes,
Sönke

P4 L26-27: seems out of place for the introduction. There is a section on EC data where this should be mentioned.

P5 L10-18: This is conclusion material, not the introduction. Merge with the Conclusion Section

P5 L26-27. This is a common place and can be removed

P6 L1-14: This is the introduction section to an extensive section of different Earth system compartments and their fluxes. No need to go into detail here. Focus on the essential (probably L9-14) and merge the remaining information with the relevant sections.

P6 L15-16. This is a common place and can be removed

P8 L7-9. This is a common place and can be removed

P9 L5: Briefly explain reviewer #2 concern here and explain why you still use the concept.

P10 L17-28: not directly relevant to the review. Shorten to make the key point.

P11 L9-10: repeats what was said in the sentences before. remove

P11 L22- P12 L16: Check for redundancies with the content of Section 2.3.1 and shorten (here or there) accordingly.

P12 L21-23: Yes, but this is not a review on the EC technique, and OCS does not circumvent this problem. Can be removed.

P13 L14-15: Is a negative repetition of the previous sentence. Remove.

P13 L19: It is not immediately clear how modeling can side-step a fundamental problem with the observations. A subclause hinting at the main information source helping to side-step this problem would be warranted.

P13 L20-22: It is a large step from resolving weekly GPP to climate carbon feedbacks. I would agree that this is a step towards better understanding the effect of synoptic meteorological variability on carbon fluxes, and therefore maybe the seasonal to interannual variability of the carbon fluxes. but not more.

P14 L2-4 is redundant with L14 16-18.

P15 L1-11: Why are such questions formulated here, whereas they aren’t in the other sections. Why is there such an emphasis on model development, when this isn’t the case for the other sections. This seems out of place to me, since in the other Sections there is also no focus on model development and future scenarios.

P19 L 4-5 “We integrate…”. Not necessary, remove

P20 L1 and following. covers abiotic processes also described in Section 2.3.4. I don’t see the need for such a duplication.

P21 L12-14. Irrelevant for a review.

Section 2.3.3. Given that not much is know, and most of the text does not concern OCS, I recommend to significantly shorten this section.

P24 L20-24: I don’t see why this is relevant here. This has nothing to do with understanding real-world OCS sorption, as becomes clear in the following sentence.

P25 L 1-10: I suggest that this can be summarized in 2 sentences and added to the atmosphere section

P25 L11-P26-L28. I think the context should be merged with the soils section and reducing redundancies significantly shortened.

P26 L18-19. Unclear whether these number are global or regional estimates. I don’t really see the need for giving these numbers here.

P30 Paragraph beginning in L26: seems more a topic for 3.1.3. Merge with information there.

Section 2.7: Misses important components of the budget (e.g. ocean, volcanoes). I would recommend moving the relevant sentences here (ie not need to repeating between 2.X and 2.7, but 2.7 should allow to understand Table 4 without having to read all of 2.X in detail. Otherwise, this section does not fit with table 4 and remains incomprehensive.

P 33 L7: There is no justification to assign the tropical biome as C4. Most tropical tree species are C3 and they contribute to a large fraction of global GPP.

P 33 L 8-10 Example of lack of adequate text editing in this manuscript. This can be significantly shortened. No need to mention Table 4 (see first sentence of Section)

P 33 L 15: Check the unit. This is a stock, not a flux. It cannot be used to calculate an OCS flux. Where does this LRU value come from?

P 33 L 17-21. Should have been part of the discussion in Section 2.2.4, and is probably partly redundant

P34 L 12 Unit missing. The last sentence is not necessary.

P35 L4: Ciais et al. 2014 missing in the reference list. Also, I don’t think that this is correct. It is a key source of uncertainty in carbon cycle prediction

P35 L11-P36 L5: This introduction is unnecessarily lengthy, as it provides information that is then repeated in the following section. Shorten as much as possible

P36 L27-P37 L10. This can be summarized more efficiently to focus on what was found. The reader can refer to Hilton et al. to see which data sources were used, and which modeling approaches were used. I also don’t understand “By using multiple estimates of each uncertainty (these are not explained)… quantitatively assessed each uncertainty source …

P37 L23-P38 L9. No need to explain the studies finding in all detail, in particular, which model did what.

P38 L8-9: This is a contradiction, please reword: A model bias cannot be only deducible by OCS, when it can also be seen in the CO2 record.

Section 3.1.2. Does this not seem a bit hopeful, given the uncertainties in the overall OCS budget and findings discussed for instance in Section 2.5?

P42 / Section 3.2. I am surprised that is has taken 42 pages to arrive at this not insignificant fact, which is largely absent from any previous discussion.

P42 L 23-P43 L9: There is no need for such level of detail in a review&synthesis. Shorten to the essentials.

P43 L 25- P45 L6: I am missing the “Application” relevance of these paragraphs. The content seems to be more relevant for Section 2, in particular the soils section. This section also misses the review and synthesis character. I recommend it to be shortened to fit into this character.

Section 4.1.: This section is disproportionally long compared to the other Sections. Please be more synthetic here. There is no need to describe individual panels of figures in this level of detail. Describe the main features and strength (to the extent that they have not been presented before).

P52 L8: This statement is not helpful, and inadequate given the other subsections in Section 4.

Section 5: This is a shopping list of things that may be interesting to do, but falls short of the aspiration of being a “community research plan”. Either write is this such that it becomes a plan, or reduce the ambition stated in the introduction.

P52 L11: This sentence is redundant with the following few sentences. remove.

P52 L 25-26. If there are not enough measurements in desert (Table 4), why do you recommend not making any measurements there?

P52 L26: Several boreal regions? This is somewhat imprecise a statement. Clarify.

P53 L9: As has been outlined very well in the preceding 53 pages, this is not as straightforward as one would have hoped. I think the presentation needs to be a bit more balanced here.

P53 L13: While I agree that such a data-base would allow to investigate global carbon / water / climate connections the data themselves will not generate a “massive advancement in our understanding”, if they are not accompanied by improved process understanding to attribute observed sources and their variability. While I do not disagree that OCS may contribute to improved understanding, I do not think that the use of the word “massive” is appropriate here. Finally, I must have missed this in the manuscript, but as far as I understand from what has been said in the manuscript, OCS may potentially help to constrain the carbon-water exchanges in terrestrial ecosystems, but this is a long way from making inferences on the effect of carbon-cycle feedbacks on the global water cycles.

P53 L20-21. Acknowledgements should be focused on contributions to the manuscript, not the overall scientific community. If you want to highlight their contributions to the field in general, this should be done in the manuscript, at an appropriate place.

many thanks for your revised manuscript, which has substantially improved over the previous version. I have a range of smaller issues that need clarification, before this manuscript can be accepted for Biogeosciences

The use of the word hydrolysis is not consistent in the manuscript. In the latter sections, you refer to hydrolysis as an abiotic process happening in water (soil or ocean). In P6 L2 you refer to hydrolysis as the major OCS dominating process in leaves, which leads me to conclude that you imply that OCS destruction in leaves was an abiotic process. This conflicts with other statements in the manuscript, and your opening paragraph in particular, that the destruction of OCS in leaves was mainly due to the involvement of enzymes controlling photosynthesis. Please clarify.

I think the section “3.1.2 Top-down budgets” is slightly misplaced as a subsection to 3.1. Certainly these problems prevent using OCS as a direct proxy for GPP. However, the larger part of this section does not talk about GPP and seems to be most relevant for Section 2.7 and the attempts to close the budget (, which of course is a requirement for estimating GPP.)

Minor issues:

P 3 L 1: (OCS, or COS, hereafter OCS)

P3 L 10: it should be noted here that there are other sources and sinks of OCS besides photosynthesis that complicate the matter.

P3 L20: explain “S”

P5 L5-7: Explain in which part of the atmosphere this trend is seen. Would it not be possible to quantify this trend in terms of concentration changes (i.e. ppt / decade) or mass? This would make the point about the inability of the souce and sink estimates to balance stronger. It would be important to explain how this trend reported here confers with the “remains in the atmosphere” entry in table 4.

P9 L18: I think it would be beneficial here to briefly introduce the following subsections so that it become clear that a more detailed treatment is following.

P20 L18: for the role in what?

P20 L22: It remains unclear what you want to conclude from this statement. Is the question not more whether the uncertainty in the ocean flux is larger or not that the gap on the overall budget?

P27 3.1: Add terrestrial

P30 L 18: Recommendation in italics.

P30 L 18: Given all the uncertainty in using OCS, I don’t think that it is accurate to state that OCS is the only constraint on these GPP variations. I think this needs to be more nuanced (e.g. promising proxy, or something alike).

Table 3 and 4 explain what “?” means

Table 4 Please also specify whether the atmosphere considered here is the sum of troposphere + stratosphere, so something else.

Figure 1: Why not adding the atmospheric imbalance from Table 4 to this figure?

Measurements of the trace gas carbonyl sulfide (OCS) are helpful in quantifying photosynthesis at previously unknowable temporal and spatial scales. While CO2 is both consumed and produced within ecosystems, OCS is mostly produced in the oceans or from specific industries, and destroyed in plant leaves in proportion to CO2. This review summarizes the advancements we have made in the understanding of OCS exchange and applications to vital ecosystem water and carbon cycle questions.

Measurements of the trace gas carbonyl sulfide (OCS) are helpful in quantifying photosynthesis...